1. Field of the Invention
This invention relates to a steel slab containing silicon for use as an electrical steel sheet and strip manufactured by continuous casting and a method for manufacturing thereof.
2. Description of the Prior Art
In recent years, electrical steel sheets and strips have been manufactured by continuous casting, and the surface defects due to blisters in the final product have been a problem.
As used herein, blister means a puffy defect on the surface of the sheet and strip caused by the expansion of gases occluded in the steel in the heat treatment of the sheet and strip. Due to the occurrence of blisters, the commercial value of the electrical sheet and strip used as a material for laminated cores is almost lost.
While it is well known that low silicon- or non-silicon electrical steel sheets and strips have been utilized as electrical sheet and strips for cores of electrical instruments in recent years, high class materials represented by grain-oriented electrical sheet and strips contain silicon, which silicon content is generally in the range of 2.5 - 4%.
On the other hand, the material contains up to 1% of Al for improving the magnetic properties.
When the Al is less than 0.04% in a high class electrical sheet and strip containing 2.5 - 4% of Si, the surface defect called blisters occur frequently. It has been ascertained that the lower the Al content, the more frequently the blister occurs and that this frequently occurs in the continuous casting material.
While the reason of the occurrence of blister has mainly been regarded as due to hydrogen occluded in the steel, this was not the conclusion reached from our studies.
The frequent occurrence of blisters in the continuous casting material as above mentioned may also be considered to be due to the influence of mold lubricant (or thermal insulator) used in the continuous casting. However, no effect from the lubricant could be observed at all in our detailed experiments.
The object of this invention is to obtain, in the manufacture of electrical sheet and strips by continuous casting, a slab having no surface defects due to blisters in producing final products.
Another object of this invention is to offer a method for the continuous casting of molten steel containing silicon whose nitrogen or oxygen content has been controlled.
A further object of this invention is to offer a continuous casting slab for producing grain-oriented electrical steel sheets and strips wherein the occurrence of surface defect due to blister is minimized.
Still further objects of this invention may be seen from the description in this specification and the attached drawings.